Greg M. Waite
Darkroom Safe Lights
Why are safe lights used in the darkroom either red or yellow? Or are there other colors that will work as well?
From "Important Facts About Safe Lights" at Kodak's
very useful site
..."In photography, the term "safelight" describes dark-room illumination that does not cause a visible change to light-sensitive material when it is correctly handled and processed. The word "safe" is relative. Most sensitized materials will be affected if you expose them to safelight illumination for an extended period of time. Because photographic materials vary in speed and sensitivity to different colors of light, the recommended bulb wattages and colors of safelight filters also vary. Safelight illumination will fog color films and papers; color print and transparency materials; most panchromatic black-and-white films (films that are sensitive to blue, green, and red light); and high-speed infrared films. You must handle these materials in total darkness.
"Ideally, safelight filters should transmit only light that is outside the color-sensitivity (wavelength) range of the photographic materials for which they are recommended. The safelight filters recommended by Kodak provide maximum transmission of colors to which the paper or film emulsion has relatively low sensitivity. However, the color sensitivity of most emulsions does not end abruptly at a particular wavelength in the spectrum. Most emulsions are somewhat sensitive to colors outside their intended range. This means that most papers and films have some sensitivity even to the colors of light transmitted by the recommended safelight filters. Therefore, always minimize the exposure of photographic materials to safelight illumination."
and specifically in answer to your question...
"The "safest" color safelight filter for a particular material is not always the recommended one. For example, a red safelight filter often has less effect on photographic papers than the amber filter listed in the table. However, most workers find that they can judge print density or perform other functions better under an amber light. So, although it is a slight compromise in protecting the paper from fogging, an amber filter improves working conditions."
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